The present invention relates to a method for forming a semiconductor structure, and more specifically to a method for forming a recess.
Recently, as fabrication techniques for semiconductor integrated circuits have developed, the number of elements in a chip has increased. Element size has decreased as integration density has increased. For example, the area of memory cells in a memory must be continuously reduced to support a larger number of memory cells, thereby increasing density. Conventional planar transistors such as metal oxide semiconductor field effect transistors (MOSFETs), however, occupy a large amount of surface area on a chip, substantially reducing the available area thereon.
Since the scalability of planar transistors in memory devices is severely limited, memory cells have looked toward utilization of vertical transistors. Vertical transistors are promising candidates for scalability, especially below minimum feature sizes of 100 nm.
Typically, as shown in FIG. 1, a memory cell with a vertical transistor requires forming a recess between two trench capacitors. The recess is defined by photolithography methods. As the feature size shrinks to 100 nm below, however, it is difficult to preciously control the location of the recess. In photolithography, a misaligned recess pattern may likely be formed on the photoresist layer due to overlay errors during exposure, causing a serious alignment shift among such as a recessed gate, an active area, deep trenches, and a bit line contact and deteriorating the yield of fabrication. This issue, however, may get more and more serious while DRAM dimension shrinking.
Thus, it is necessary to develop a method providing a precise alignment control among semiconductor elements, in particular, the distances between a recessed gate and deep trenches.